The present invention relates to a method of removing moisture in a gas supply system used in the manufacture of semiconductors, chemicals, precision machine parts and the like. More particularly, the present invention relates to a method of removing moisture in the gas supply system effectively by adjusting the flow pressure of the gas for removing moisture within a specific range without baking (heating) the gas supply system or by intermittently supplying the gas for removing moisture at specific intervals.
Various kinds of high-purity gases are generally supplied to a gas supply system in semiconductor manufacturing facilities, chemical manufacturing facilities and the like. If semiconductors, chemicals and the like are to be manufactured with high efficiency and purity, it is essential to keep impurities from getting into high-purity gases.
However, when the gas supply system is opened to the atmosphere as in the inspection of the facilities and the suspension of manufacture, air, moisture and other impurities flow into the system.
Those impurity gases are adsorbed on the inside surface of the piping and other component parts such as various valves and filters. The filter in particular is large in adsorption area and the adsorbed impurity molecules are difficult to remove.
Previously, impurity gases adsorbed inside the gas supply system were removed by purging the inside of the gas supply system with a high-purity gas for a long time, or baking the gas supply stem from outside.
Since an H2O molecule in particular is stronger than other adsorbed molecules in adsorbability, baking is considered to be the most effective way of removing impurity gases.
Whether the impurity gases have been removed is generally judged in a way that the main components in the evacuated gas are compared before and after the baking. The largest impurity gas before baking is H2O. In baking, water decreases drastically as compared with other gases. In other words, moisture is a gas very difficult to remove by evacuation at normal temperature, but is easy to remove by baking.
However, baking often has adverse effects on the gas supply system. For example, because of high temperature, materials deteriorate in physical properties. Due to an increase in diffusion coefficient in the solid, segregation, thermal decomposition, or the like, occur. In particular, the gas supply system itself is often so designed that it should not be baked. In such a case, the system cannot be baked.
An general object of the present invention is to provide a method of removing impurity gases efficiently without baking. A first specific object of the present invention is to provide a method of removing moisture in a gas supply system in which adsorbed moisture can be removed effectively by evacuation at normal temperature.
A second specific object of the present invention is to provide a method of removing moisture in the gas supply system in which adsorbed moisture can be effectively removed by maintaining the flow pressure of the gas for removing moisture within a specific range.
A third specific object of the present invention is to provide a method of removing moisture in the gas supply system in which adsorbed moisture can be removed efficiently by evacuation at normal temperature even if the gas supply system is a complicated one having valves, filters, pressure regulators, flow regulators, etc.
A fourth object of the present invention is to provide a method of removing moisture in the gas supply system in which moisture can be removed effectively by evacuation at normal temperature even in a gas supply system of a complicated construction by stopping and allowing, at specific intervals, the flow of the moisture-removing gas without regulating the flow rate of the gas for removing moisture in a meticulous manner.
The present invention solves the above problems. As described more fully below, the present invention is to a method of removing remaining moisture in a gas supply system in which the flow pressure of the gas for removing moisture is set at not lower than a minimum pressure where the gas flow becomes a viscous flow and not higher than a water-saturated vapor pressure, which is the basic principle of the present invention.
A feature of the present invention is that a condition allowing the gas for removing moisture to be a viscous flow is that the mean free path of gas molecules be smaller than the diameter of the piping of the gas supply system.
The gas supply system from which moisture is removed may be either a gas supply system of a relatively simple construction having filters and valves or one of a relatively complicated construction having filters, pressure regulators, flow regulators, valves, etc.
Also, the gas supply system may be a gas supply system provided with an evacuation port for the gas for removing moisture at an end of the gas supply system and another on an upstream side of the flow regulator.
Meanwhile, the basic principle of the present invention is a method of removing moisture remaining in the gas supply system by flowing the gas for removing moisture in the gas supply system in which the inside of the gas supply system is evacuated continuously and at the same time the moisture-removing gas to be supplied into the gas supply system is stopped and allowed to flow at specific intervals.
While the moisture removing gas to be supplied into the gas supply system is stopped, moisture is removed with high efficiency by evacuating to bring the flow pressure of the gas for removing moisture in the gas supply system to a pressure value between the minimum pressure where the gas flow becomes a viscous flow and the water saturated vapor pressure at the flow temperature of the gas for moving moisture.
The supply pressure of the moisture removing gas to be supplied in the gas supply system is brought to between 100-4,500 Torr. As a result, while the supply of gas for removing moisture is stopped, the flow of the gas for removing moisture in the gas supply system is a viscous flow without fail and the gas pressure in the gas supply system is not higher than the water vapor pressure in the gas supply system is not higher than the water vapor pressure, thereby facilitating the desorption and removal of the adsorbed water molecules efficiently. Thus, the desorbed water molecules are driven out by the supply of the gas for removing moisture that follows, and the moisture is removed more efficiently.
Further, the moisture removing gas from the supply source can be supplied into the gas supply system without adjusting the gas flow rate on a primary side of the gas supply system. This further simplifies the moisture removing process.
The moisture-removing gas to be supplied into the gas supply system is allowed to flow preferably for 0.9 to 5 seconds and is stopped for 0.3 to 175 seconds alternately. This further improves the moisture-removing efficiency.